He can't use both at the same time.
S10 means he's using the doomfists in CC, which would mean he can't use the sword.
So he has to choose between S10 or re-rolls on hits/wounds/armour pen.
Dreadknights are tough, but really, just treat them like a tank, and they'll go down accordingly.
They're also a crapload of points if you give them any upgrades, which is something GK have to watch very carefully.
So, a potentially tough unit, yes, but nothing that should be ruining your day consistently. Play more against them and I'm sure you'll do fine.